The use of the electricity network as a means of communication is know in the prior art, but due to its poor performance, its use as a data transmission network has been limited to point-to-point communication at very low speeds.
This is due, among other reasons, to the fact that in the electricity network the connection and disconnection of apparatus generate voltage peaks and impedance variations on the line causing serious loss of signal that varies in function of frequency and time.
Furthermore, various obstacles impede the establishment of communication between a head-end and multiple users, due to the many impedance changes in the different frequencies and the production of reflections that cause the signal received to be a combination of the transmitted signal and a series of echoes that circulate through the electricity network with different attenuations and delays for each one of the users on the received signal.
Furthermore, attenuation, noise, and channel response vary dynamically both in frequency and in time.
All these obstacles limited the use of the electricity network for full-duplex, high-speed point to multipoint communication, until the appearance of the patent P-200003024, as referred to previously, that supports a system where various user kits and a head-end are in two-way communication via the electricity network, one channel being the upstream from the users to the head-end, and the other the downstream going from the head-end to the user kits, including a medium access control module (MAC) in each one of the kits to maximize the quantity of information that the user kits can transmit and minimize time latency in these user kits while the division of the electricity network for the upstream and downstream channels is made by means of duplexing by division in frequency and/or by means of duplexing by division in time and where both head-end and user kits include the means to adapt the corresponding digital transmission to the electricity network.
The system supported by the patent P-200003024 as already mentioned, adequately resolves the inconveniences referred to previously, being nevertheless capable of incorporating diverse processes, among which is the process described in the present invention.
On the other hand, other means of communication for the transmission of data are known in the background art, such as the use of the twisted pair in telephones to establish point-to-point or point to multipoint communication.
In this context we cite U.S. Pat. No. 5,673,290 wherein a method of transmission point-to-point is described that consists of communication via a downstream channel determined by a link from the head-end to various different users, and communication via the upstream channel determined by a link from the user to the head-end, whereby the communication is made possible using a discrete digital multi tone (DMT) transmission system and providing the coding of the digital data and the modulation of the codified data over the discrete multi-tone signal.
Furthermore, the communication line is supervised to determine at least one line quality parameter, including noise levels in each one, and includes a multitude of subchannels each one corresponding to an associated sub carrier tone. The modulation system is designed to take various factors into account including detected line quality parameters, the parameters of sub-channel gains, and a masking parameter of permissible power when modulating the discrete multi tone signal. The modulation system is also capable of dynamically updating the subcarriers used and the quantity of data transmitted in each sub carrier during transmission to adapt in real time to changes in individual subcarriers.
In applications susceptible to interference, the associated bandwidths can be simply masked or silenced to prevent interference in either direction, and therefore, the signals are transmitted by subcarriers with frequencies above or below the most significant noise levels.
Furthermore, in this U.S. Pat. No. 5,673,290 the transmission occurs in base band and the conjugated real hermitian transformation of the transmissible information is used (real Fast Fourier Transform). Due to the characteristics already described, this transmission method cannot be applied to transmission over the electricity network.
Furthermore, the method described in this US Patent refers to point-to-point communication, therefore, neither its use over the electricity network nor the possibility for full duplex point to multipoint communication can be inferred.
On the other hand, point to multipoint communication systems exist such as that described in the PCT Patent Number WO96/37062 where the transmission line can be coaxial cable, fibre optic or similar, which use orthogonal frequency division multiple access modulation system (OFDM), a modulation system that is well known in the background art, and to which a cyclic prefix is added to each OFDM symbol to alleviate the defects of the multi path propagation as is well known in the state of the art. The use of the cyclic prefix with the OFDM modulation can be encompassed by the DMT modulation used in the previous document and is also widely used in the state of the art.
This PCT document describes how channels are established over respective sub-carrier groups, so that each user is assigned a specific group of tones so that the hardware and the complexity involved in realizing the discrete Fourier transformation is substantially reduced, however, as a fixed system it does not allow the assignation of different subcarriers to the users depending on the prevailing frequency and time conditions in each channel, even when, as described in the case of U.S. Pat. No. 5,673,290, the individual subcarriers can be connected or disconnected to avoid interferences.
Furthermore, it uses a remote loop to correct the frequency of local oscillators of the various user modems.
We can also cite U.S. Pat. No. 5,815,488 and U.S. Pat. No. 5,828,660 regarding point to multipoint communication.
These documents do not have a description of the adaptation for the transmission using the electricity network.
Furthermore, none of the documents previously cited concern the transmission for multiple users, or how to maximize the throughput of the upstream and downstream channels in the electricity network.